This invention relates to a control system for stopping a spindle at a predetermined rotational position, and more particularly to a control system of the type described, which permits the control operation to be performed in a short period of time by rendering the positioning gain approximately constant regardless of the speed change ratio, between the spindle and a motor, as determined by a speed change mechanism.
In an automatic tool change mechanism of a machine tool, a key is provided on the spindle of the machine tool and a keyway is formed in each of the various tools that are employed. In order to mate the spindle and tool with each other smoothly, the spindle must be positioned and stopped accurately in such a manner as to bring the key and keyway into perfect alignment. There is a similar requirement in machine tools for cutting work, and for boring work in particular. Here it is desirable to employ a boring bar having a large diameter in order to eliminate chatter, and to stop a specified point on the spindle at a predetermined rotational position in an accurate manner in order that the cutting operation may proceed in a stable and rigid fashion.
Mechanical stopping mechanisms that rely upon a pin or brake are generally employed to stop the spindle at the predetermined position. However, such mechanisms experience wear with long use, particularly of the pin and brake shoe portions, so that it becomes progressively more difficult to stop the spindle at the correct position. These mechanisms also require troublesome maintenance and inspections.
Accordingly, the assignee of present invention has already proposed a system, disclosed in copending U.S. applications by Kohzai et al. with Ser. No. 190,659 entitled "Spindle Rotation Control System" and by Fujioka with Ser. No. 215,631 entitled "Control System for Stopping Spindle at Predetermined Rotational Position" both assigned to the assignee of the present application, for stopping a specified point on spindle at a predetermined rotational position through purely electrical means without relying upon a mechanical brake or the like. Although the already proposed system succeeds in stopping the spindle with a high degree of accuracy, a considerable amount of time may be required to accomplish the stopping if the speed change ratio between the spindle and the spindle driving motor is high. This naturally results in a poorer working efficiency.
Approximately 90% of the machine tools of the ordinary variety employ a speed change mechanism, such as a gear mechanism between the motor and spindle, so that machining can be performed while switching between a high gear setting (reduction ratio low) and a low gear setting (reduction ratio high). The high gear setting is used for the light machining of wood or light metals such as aluminum, whereas the low gear setting is employed for the heavy machining of steel or the like.
In the already proposed control system for stopping a spindle at a predetermined rotational position, the control mode for stopping the spindle begins after the generation of a single for one spindle revolution, with the specified point on the spindle being stopped correctly at the predetermined position after the one revolution. To accomplish this the gear ratio between the spindle and motor is set to high gear (1:1) to enable the spindle to be stopped after a single revolution of the motor. In the low gear setting, however, where the motor-to-spindle ratio is typically 4:1, four revolutions of the motor are required to bring the spindle to a stop. Thus the time required for the stopping operation in low gear is at least four times that of high gear, so that the proportion of time spent for actual machining work decreases. The inevitable result is a decline in working efficiency.
In general, with the high gear setting in which the gear ratio is 1:1, load inertia increases and speed loop gain decreases, while the opposite is true in the low gear setting in which the gear ratio is 4:1, as mentioned above. In other words, when applying the orientation control operation to stop the spindle at the predetermined position, the low gear setting allows the motor speed to be increased in comparison with the speed under a high gear setting.